Photovoltaic solar cells are semiconductor devices which convert sunlight into electricity. Solar cells based on crystalline silicon offer the advantage of high performance and stability. The principal barrier to expanded utilization of silicon solar cells for electric power generation is the present high cost of the solar cells.
Significant cost reductions can be achieved by using thin-film solar cells. Thin-film solar cells use thin layers of semiconductor materials that are deposited or grown on low cost substrates. A publication by A. M. Barnett et al. in the Conference Record of 19th IEEE Photovoltaic Specialists Conference (1987) pp. 1266-1270, which is incorporated by reference herein, describes polycrystalline silicon thin-film solar cells on low cost substrates. In addition to the lower cost that results from using less photovoltaic grade silicon, i.e., less than a 100 micron thick layer instead of a 400 micron thick wafer, improved performance is also predicted as a consequence of the thinness of the silicon. The publication describes further enhancement in performance that results from employing optical confinement techniques. In a related publication, which appears in the Technical Digest of the Third International Photovoltaic Science and Engineering Conference, PVSEC-3 (1987) pp. 101-104 and is incorporated by reference herein, A. M. Barnett et al. disclose a set of design criteria for thin film silicon to yield high performance solar cells.
The prior art includes a number of descriptions of thin silicon solar cells. For example, U.S. Pat. No. 4,571,488 issued to A. M. Barnett discloses a thin film solar cell formed on various substrate materials and a conducting or semiconducting metallurgical barrier layer between the substrate on the active layers of the solar cell. M. G. Mauk et al. in the Conference Record of the 18th IEEE Photovoltaics Specialists Conference (1985) pp. 192-197, describe a solar cell with thin epitaxial silicon active layers on a heavily doped silicon wafer with a thin film of silicon dioxide between the active layers and the substrate. U.S. Pat. No. 4,677,250 to A. M. Barnett et al., discloses solar cells employing polycrystalline thin-film semiconductor active layers deposited on various substrates and an insulator with conductive nucleation sites between the substrate and the active semiconductor layer.
It is an object of this invention to provide a low cost, high efficiency thin silicon solar cell which is formed on a low cost substrate.